The present invention relates to x-ray imaging and, more particularly, to calibrating an x-ray laminography imaging system that utilizes a stationary x-ray source and generates a moving pattern of x-ray spots on a target to reduce or eliminate the need to move the object being imaged.
Laminography techniques are widely used to produce cross-section images of selected planes within objects. Conventional laminography requires a coordinated motion of any two of three main components comprising a laminography system (i.e., a radiation source, an object being inspected, and a detector). The coordinated motion of the two components can be in any of a variety of patterns, including linear, circular, elliptical and random patterns. Regardless of the pattern of coordinated motion selected, the configuration of the source, object and detector is such that any point in the object plane (i.e., the focal plane within the object) is always projected to the same point in the image plane (i.e., the plane of the detector), and any point outside the object plane is projected to a plurality of points in the image plane during a cycle of the pattern motion. In this manner, a cross-section image of the desired plane within the object is formed on the detector. The images of other planes within the object experience movement with respect to the detector thus creating a blur, i.e. background, on the detector upon which is superimposed the sharp cross-section image of the focal plane within the object. This technique results in sharp images of the desired object focal plane. Although any pattern of coordinated motion can be used, circular patterns are generally preferred because they are more easily produced.
The laminography techniques described above are currently used in a wide range of applications including medical and industrial X-ray imaging. Laminography is particularly well suited for inspecting objects that comprise several layers, with each layer having distinguishable features. However, laminography systems that produce such cross-sectional images typically experience shortcomings in resolution and/or speed of inspection, thus accounting for its rare implementation. These shortcomings are frequently due to the difficulties in achieving high speed coordinated motion of the source and detector to a degree of precision sufficient to produce a high resolution cross-section image.
In a laminography system having a field of view that is smaller than the object being inspected, it may be necessary to move the object around within the field of view to obtain multiple laminographs which, when pieced together, cover the entire object. Movement of the object is frequently achieved by supporting the object on a mechanical handling system, such as an X, Y, Z positioning table. The table is then moved to bring the desired portions of the object into the field of view. Movement in the X and Y directions locates the area to be examined, while movement in the Z directions moves the object up and down to select the plane within the object where the image is to be taken. While this method effectively enables various areas and planes of the object to be viewed, there are inherent limitations associated with the speed and accuracy of such mechanical motions. These constraints effectively act to increase cycle time, thereby reducing the rates at which inspection can occur. Furthermore, these mechanical motions produce vibrations which tend to reduce the system resolution and accuracy.
In order to reduce or eliminate the need to move the object, and the problems associated therewith, an off-axis laminography system has been invented, which is disclosed in U.S. Pat. No. 5,259,012 (the ""012 patent) and which is incorporated herein by reference in its entirety. The ""012 patent discloses a laminography system in which off-axis scanning circles can be used to enable multiple locations on an object to be sequentially imaged without requiring mechanical movement of the object. The phrase xe2x80x9coff-axisxe2x80x9d refers to placing the center of the scan circle in a position that is not concentric with the optical axis of the imaging system. In the imaging system disclosed in the ""012 patent, x-rays are produced when highly accelerated electrons impinge on a metal target. The point where the x-rays are produced is commonly referred to as the xe2x80x9cspotxe2x80x9d. The spot can be steered across the target by electronically controlled deflection coils which act on the electron beam. Moving the scan patterns (i.e., the pattern of spots) produces laminographs at desired X, Y coordinate locations with various Z planes and generally reduces or eliminates the need to mechanically move the object.
The ""012 patent discloses an x-ray source that includes an electron gun that emits an electron beam. The electron beam is incident upon a flat target anode (hereinafter referred to as xe2x80x9cthe targetxe2x80x9d). Focus and deflection coils direct the electron beam to specific locations on the target to form the aforementioned circular electron beam patterns on the surface of the target. When the electrons are slowed down or stopped in the target, Bremsstrahlung x-rays are generated. Since the electron beam describes a moving circular pattern on the target, the source of Bremsstrahlung x-rays also describes a moving circular pattern coincident with the electron beam pattern. In one embodiment of the ""012 patent, steering signals applied to the deflection coils cause the electron beam spot to rotate in a predetermined path in coordination with a similar path of the detector. In an embodiment, a digital look-up-table (LUT) sends digital signals to the deflection coils that cause the beam spot to follow the circular motion of the electron beam on the target. In the latter case, digital addresses corresponding to the location of the x-ray detector along the circle traced by the detector are sent from the detector to the LUT. The LUT then sends deflection signals corresponding to specific detector positions to the electron beam deflection coils. The values of the deflection signals are calibrated to cause the x-ray source to trace a circular pattern upon the target that is precisely coordinated with the motion of the detector.
Current laminography calibration techniques trace the circular patterns on the target and gather empirical data for each circular pattern. The empirical data for each circular pattern is then processed to generate the LUT values needed to reproduce the circular pattern at run time. This type of calibration technique is suitable for on-axis laminography because the number of circular scan patterns is not too great (e.g., N scan circles for N different magnifications, where N=4 for typical existing implementations). Therefore, calibrating the system to obtain the appropriate LUT values may take, for example, one to four hours (depending on the level of magnification). However, when off-axis laminography is employed, the number of scan circles that must be generated to cover the object area of interest increases significantly due to the number of different off-axis positions. Thus, depending on the size of the scan circles that are desired and the target area available, the number of scan circles needed to image the object may increase significantly when performing off-axis laminography. If the aforementioned calibration technique is employed for an off-axis laminography system, these numbers suggest that multiple days may be required to calibrate the system for off-axis imaging at multiple magnifications. Calibration times of this length are generally unacceptable to users.
Accordingly, a need exists for a calibration method and apparatus that are suitable for use with on-axis and off-axis laminography and that enable calibration to be performed in a relatively short amount of time.
The present invention provides an x-ray laminography imaging system that utilizes a stationary x-ray source and generates a moving pattern of x-ray spots on a target anode synchronously with rotation of an x-ray detector to eliminate the need to move an object being imaged. The present invention provides an apparatus and a method for calibrating the system based in part on empirical data gathered during physical calibration and in part on data analytically derived from the empirical data. Because calibration of the system can be performed in great part analytically rather than relying entirely on empirically generated data, the calibration process can be performed very quickly.
The apparatus comprises first logic, second logic and third logic, which preferably correspond to a processor configured to execute a calibration algorithm. The first logic is configured to gather empirical calibration data generated during physical calibration of the system during which a stationary x-ray source generates a moving pattern of x-ray spots on a target anode synchronously with rotation of an x-ray detector. The empirical data corresponds to offsets to locations at which the x-ray spots should be formed on the target anode. The second logic is configured to analytically derive calibration data from the empirical data, preferably by interpolation. The third logic is configured to calibrate the system using the empirical data and the analytically-derived calibration data.
The system comprises a controllable deflection yoke that receives control signals from a processor. The controllable deflection yoke controls particular locations on the target anode upon which x-rays projected by an x-ray source along a Z-axis impinge in accordance with the control signals received. The target anode is oriented substantially parallel to a plane that is substantially orthogonal to the Z-axis. The x-rays projected along the Z-axis impinge at particular locations on the target anode that are dependent on control signals received by the controllable deflection yoke. The x-rays directed onto the target anode form substantially circular x-ray spot patterns on the target anode, with each x-ray spot pattern being produced by movement of an x-ray spot in a substantially circular pattern. Each x-ray spot corresponds to a beam of x-rays impinging on a particular location on the target anode. The control signals cause the deflection yoke to form at least one substantially circular on-axis x-ray spot pattern on the target anode about the Z-axis and at least one substantially circular off-axis x-ray spot pattern on the target anode about an axis that is substantially parallel to the Z-axis. The processor determines the control signals needed to be delivered to the deflection yoke to cause the off-axis x-ray spot pattern to be formed based on data associated with the on-axis x-ray spot pattern.
The determination by the processor of where on the target anode the x-ray spots of the off-axis x-ray patterns are to be formed is analytically made using the empirically-generated offset on-axis x-ray patterns. From a relatively small number of empirically-generated offset on-axis patterns, a large number of off-axis x-ray patterns can be generated and used to calibrate the system.
The method of the present invention comprises the following steps: determining control signals needed to be delivered to a deflection yoke to cause at least one substantially circular on-axis x-ray spot pattern to be formed on a target anode about a Z-axis to simulate rotation of an x-ray source; processing data gathered through calibration of the system as a rotating x-ray detector is synchronized to the motion of the x-ray spots about the Z-axis that form the on-axis x-ray spot pattern to determine offsets to the X, Y coordinates of the x-ray spots of the pattern on the target anode; using the offsets to offset the X, Y coordinates of the x-ray spots of the on-axis x-ray spot pattern as they x-ray spot pattern is being formed on the target anode, thereby causing an offset on-axis x-ray spot pattern to be formed on the target anode about the Z-axis; and using the x-ray spot offsets associated with the on-axis x-ray spot pattern to determine a substantially circular off-axis x-ray spot pattern to be formed on the target anode about an axis that is substantially parallel to the Z-axis.
These and other features and advantages of the present invention will become apparent from the following description, drawings and claims.